Brushless direct current (BLDC) motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, high dynamic response, high efficiency, long operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. More detailed information on BLDC motors may be found in Microchip Application Notes: AN857, entitled “Brushless DC Motor. Control Made Easy,” (2002); AN885, entitled “Brushless DC (BLDC) Motor Fundamentals,” (2003); AN894, entitled “Motor Control Sensor Feedback Circuits,” (2003); AN901, entitled “Using the dsPIC30F for Sensorless BLDC Control,” (2004); and AN970, entitled “Using the PIC18F2431 for Sensorless BLDC Motor Control,” (2005); all are hereby incorporated by reference herein for all purposes.
A three-phase BLDC motor completes an electrical cycle, i.e., 360 electrical degrees of rotation, in six steps at 60 electrical degrees per step. Synchronously at every 60 electrical degrees, phase current switching is updated (commutation). However, one electrical cycle may not correspond to one mechanical revolution (360 mechanical degrees) of the motor rotor. The number of electrical cycles to be repeated to complete one mechanical revolution depends upon the number of rotor pole pairs. For example, a four-pole BLDC motor will require two electrical cycles to complete one mechanical revolution of the motor rotor (see FIG. 3).
Drive commutation for a sensorless BLDC motor may also be determined by monitoring the back electromotive force (EMF) voltages at each phase (A-B-C) of the motor. The drive commutation is synchronized with the motor when the back EMF of the un-driven phase crosses one-half of the motor supply voltage during a commutation period. This is referred to as “zero-crossing” where the back EMF is equal to one-half of the motor supply voltage, over each electrical cycle. Zero-crossing is detected on the un-driven phase when the drive voltage is being applied to the driven phases. A voltage polarity change about the zero-crossing voltage of the back EMF on the un-driven phase may also be used in detecting a zero-crossing event, e.g., from positive to negative or negative to positive during application of the drive voltage to the driven phases within certain limits.
The rotational speed of a BLDC motor is dependent upon the amplitude of the average DC voltages applied to the stator windings of the motor. The higher the average DC voltage applied, the faster will the BLDC motor rotate. Generally, DC voltages are generated using pulse width modulation (PWM) to control the voltage amplitudes applied to the stator windings. The PWM maximum frequency is limited by the switching losses of the drive transistors. The PWM minimum frequency is limited by the undesirable audio emissions at frequencies in the audio range. An acceptable compromise is in the 15 KHz to 20 KHz range. PWM duty cycle can only be reduced to the point where the drive pulse width can still propagate through the drive power field effect transistors (FETs) and low-pass filter characteristics inherent in all motor designs. Reducing the PWM frequency would allow longer drive periods but this would also introduce audible noise from the motor. Every PWM signal pulse requires power switching transistors, e.g., power field effect transistors (FETS), to turn on and off. Rapidly turning power switching transistors on and off creates power losses as the transistors go from an off-condition through a voltage/current transition to an on-condition. Typically, power losses are low when the power switching transistor is saturated to full conduction in the on-state, and substantially no current flow in the off-state. Getting power switching transistors from the off-state to the on-state and visa-versa, creates significant power losses in these transistors. The more the power transistors are switched between the on and off states, the greater the power losses and power dissipation in the power transistors.
Motor drive switching losses in the power transistors at high power loads are exacerbated because power dissipated therein is the square of the current times the increased resistance during the on-off-on-off transition times. As drive current increases the power dissipated in the power switching transistors increase exponentially. Gaps in drive voltage caused by the PWM off periods create timing errors in sensorless zero cross detection. Furthermore PWM signal generation at high frequencies lead to inefficiencies of the power switching transistors and failures due to overheating of the power switching transistors.